The present invention relates to a distance measuring method and a distance measuring device for measuring a distance by projecting a laser beam as a distance measuring light to an object to be measured and by receiving a reflection light from the object to be measured. In particular, the present invention relates to a distance measuring method and a distance measuring device for projecting the distance measuring light for scanning.
When a distance to an object to be measured is measured by projecting a laser beam as a distance measuring light toward the object to be measured and by using a reflected light from the object to be measured, two methods are known as a distance measuring method: a collimation type electro-optical distance measuring method for performing distance measurement to the object to be measured on 1:1 basis, and a scanning type electro-optical distance measuring method for performing distance measurement by receiving a reflection light from the object to be measured while projecting a distance measuring light for scanning.
When a laser beam is emitted from a light source, there is intensity distribution normally within a light emission plane due to interference. As shown in FIG. 10, in a cross-section 3′ of a distance measuring luminous flux of the laser beam, i.e. a distance measuring light 3, speckle patterns 4 occur due to the intensity distribution. Therefore, the projected distance measuring light 3 also contains speckle patterns 4 caused by the intensity distribution. For this reason, when a distance is measured according to a reflected light from the center of the luminous flux of the laser beam projected to the object to be measured or when a distance is measured according to a reflected light from the edge region of the luminous flux, difference occurs in the measured distance due to the speckle patterns.
In the collimation type electro-optical distance measuring method as described above, measurement is made by collimating the central portion of the luminous flux of the distance measuring light. As a result, it is a measurement in a fixed condition, and the measurement can be repeatedly performed. Therefore, the values of the measured distance are averaged, and this makes it possible to reduce fluctuation of the values of measured distance caused by the speckle patterns.
As disclosed in the Japanese Patent Publication No. 2580148 or in JP-A-2000-162517, the influence of the speckle pattern is eliminated by homogenizing phase unevenness and light intensity unevenness of the distance measuring luminous flux.
On the other hand, according to the scanning type electro-optical distance measuring method, the distance measuring light 3 moves with respect to the object 2 to be measured as shown in FIG. 11, and the center of the object 2 to be measured is not always at the center of the cross-section 3′ of the distance measuring luminous flux. The object 2 to be measured may be in the edge region of the distance measuring luminous flux cross-section 3′. As a result, when the distance is measured, the results of the measurement may include the result of distance measurement at the center of the distance measuring luminous flux and the result of distance measurement at the edge region of the distance measuring luminous flux. FIG. 11 shows a prism for distance measurement installed on a pole 1 as an object 2 to be measured, and the distance measuring light 3 is projected to scan within a scanning surface 5.
When a distance is measured in the edge region of the distance measuring luminous flux cross-section 3′, a weighted position varies when the distance measuring light is detected according to the light amount in the edge region and due to the influence of speckle patterns. As a result, deviation occurs in the timing of detection.
An electro-optical distance measuring device measures a distance according to a phase difference between a reflected distance measuring light 6 and an internal reference light or according to the time obtained from deviation in time. If there is phase deviation in the reflected distance measuring light 6, serious error may occur in the value of the measured distance.
Because the distance measuring light 3 is continuously projected for scanning, measurement cannot be repeatedly performed under condition that the object 2 to be measured is collimated, and the influence of the speckle pattern cannot be excluded. Therefore, when distance is measured by the scanning type electro-optical distance measuring method, measurement is made for one time based on partial reflection from the object to be measured of the single distance measuring luminous flux, and the measured distance value is under the influence of the speckle pattern of the distance measuring luminous flux.
Further, if the distance measuring light used for distance measurement does not have a certain predetermined light intensity, S/N ratio of the reflected light from the object to be measured is decreased, and this results in the problem of lower measurement accuracy. In case the measurement can be performed repeatedly, the measured values of the repeated measurement can be averaged, and this increases the measurement accuracy. However, in the scanning type electro-optical distance measurement with a single measuring operation, it is necessary to increase light intensity (light amount) of the distance measuring light.